This invention relates to a programmable read-only memory having a programming interface circuit requiring a reduced number of input signals.
Programmable read-only memories (also referred to as PROMs), such as one-time programmable read-only memories, erasable programmable read-only memories (EPROMs), and electrically erasable programmable read-only memories (EEPROMs), are convenient for storing data and programs in electronic devices, because they can be programmed by the manufacturer of the device rather than by the manufacturer of the memory. When a PROM is embodied as an individual semiconductor device, it is programmed by writing data via external terminals connected to an internal data bus and address bus, and supplying power and control signals via other external terminals.
When a PROM is combined with another functional circuit such as a microprocessor or speech synthesizer in a single semiconductor device, however, the other functional circuit has its own external terminal requirements. If separate external terminals were used for the PROM address and data buses, the total number of external terminals would become prohibitively large, causing problems of difficult mounting and poor reliability.
The inventor has, accordingly, disclosed a scheme whereby the address bus and data bus of the PROM are not connected to external terminals. Such a PROM has an interface circuit enabling the PROM to be read by another functional circuit in the same device, and to be programmed using comparatively few external terminals. In a typical prior-art device these external terminals include a Vss (ground) terminal, a Vcc (supply voltage) terminal, a Vpp (programming voltage) terminal, and seven signal terminals: a CE (chip enable) terminal, an OE (output enable) terminal, an SCK (serial clock) terminal, an EQL (equal) terminal, a DATA (data) terminal, a DOE (data output enable) terminal, and an LS (latch signal) terminal.
It would be desirable, however, to reduce the number of external signal terminals devoted to the PROM to an even smaller number than seven, both to save space and to improve the reliability of the device of which the PROM is a part.